Rising ocean surface temperatures caused by global warming have increased the temperature and moisture content of the air passing over the United States, thus setting the stage for heavier snow and rainstorms.

A few snowstorms, cold snaps or even heat waves do not prove anything about climate change, because there is a significant difference between weather and climate. Weather is what we experience on any given day or even over a couple of weeks. Climate describes a region’s prevailing conditions — including such things as temperature, rainfall, wind, humidity and atmospheric pressure — over long periods of time. Climate is a good indicator of what to expect. For example, in the Midwest, one would expect cold winters, whereas in a Mediterranean climate, one would expect a generally milder winter.

Climate change refers to shifts in prevailing conditions observed over decades. One such shift is a long-term rise in global average temperatures. The current cold spells are occurring against this backdrop.

Putting aside the difference between weather and climate, climate change projections show that a warming planet generates more precipitation in areas that typically experience rain or snow. Rising ocean surface temperatures already have increased the temperature and moisture content of the air passing over the United States, setting the stage for heavier snow and rainstorms. An Intergovernmental Panel on Climate Change report found that global warming has increased the frequency of storms that dump heavy precipitation over most land regions that experience storms. Most deserts, conversely, are getting drier.

“Climate scientists aren’t at all surprised that there are more drenching rain or blizzards in certain parts of the country,” says Dr. Brenda Ekwurzel, a climate scientist with the Union of Concerned Scientists (UCS). “That’s consistent with well-documented climate change trends over the past several decades. Unless we take some dramatic steps to curb global warming, we likely will see a lot more regional precipitation over the next few decades.”

Precipitation in the Northeast has increased markedly over the last century, according to the Northeast Climate Impact Assessment, a collaboration between UCS and a team of more than 50 scientists and economists. Over the past few decades, winter precipitation in the Northeast has increased 0.15 inch per decade.

The Northeast is not alone. According to Global Climate Change Impacts in the United States, released last year by 13 federal agencies, Great Lakes states are experiencing more precipitation because the lakes have less ice and more open water in the winter. The maximum seasonal coverage of Great Lakes ice decreased approximately 30 percent from 1973 through 2008. That means more lake water is likely to evaporate into the atmosphere, resulting in heavier snowstorms.

The Union of Concerned Scientists is the leading science-based nonprofit organization working for a healthy environment and a safer world. Founded in 1969, UCS is headquartered in Cambridge, Mass., and has offices in Berkeley, Calif., Chicago and Washington, D.C.